A complaint often heard is that Jesus was wrong about his soon return. A principle passage that skeptics refer to is Mark 13 where they declare "this generation" means the generation of the disciples, and "you" is a direct address to the disciples. Since Jesus did not return in the time period implied, he got it wrong.
Let's take Mark 13 and ask what Jesus meant by "this generation."
Mark 13:29,30. 29 Even so, when you see these things happening, you know that it is near, right at the door. 30 Truly I tell you, this generation will certainly not pass away until all these things have happened.
The key phrase to understanding is the clause "when you see these things happening." That is the clue to understanding what generation Jesus had in mind. He used the illustration of the fig tree leafing as a sign that summer is near (v.28). When you see the tree leafing you know summer is near. "When you see:" that is the generation.
What things are those that Jesus told us to look for that would indicate that the end is near?
Verse 26 and 27 are basically describing his coming: 26 Then they will see the Son of Man coming in clouds with great power and glory. 27 And then He will send forth the angels, and will gather together His elect from the four winds, from the farthest end of the earth to the farthest end of heaven. It will be obvious. No secret coming here.
Working our way back, Verses 24, 25 are a quote from Isaiah 13:10 and 34:4. Both have to do with a future judgment of the world called the Day of the Lord. It is following this Day of the Lord that Jesus will return in power and glory (26,27). These events have not yet happened. They did not happen in the life times of the disciples.
Verses 20-23 warn of false prophets arising claiming to be messiahs. They will do signs and miracles. Maybe we could say that these false prophets are among us now but false messiah doing signs and wonders is nowhere described in any report of the first century or within the life times of the Apostles.
Now to the beginning of the passage (v.14): It begins with a reference back to something that will happen before the events of verses 20-27 unfold.
What is the "something that will happen before?" It is the desecration of the temple (reference to Daniel 9, 11, 12) and the warning to those in Jerusalem to flee. Did this happen in 70 AD when the temple was destroyed by the Romans? Perhaps. And that event did happen during the life time of some of the disciples. But there is no record of anything like what Daniel predicted. In Daniel 9:27 there is a prediction of a covenant made and then broken and something that desecrates the temple is set up. Neither of these things happened in the Jewish Roman war. This desecration of the temple must be something later. And since the temple was completely destroyed in 70 AD there must be another temple that will take its place. That hasn't happened yet, though some Jews are making plans.
Back further, verses 12,13 speak of brother betraying brother and everyone hating them (the "you"). Has that happened? Perhaps, but it seems more intense here than what Christians (the "you") or the disciples ( the possible "you") experienced.
Back further, verse 9-11 speak of the gospel being preached to all nations before the end will come (v. 10). Had that happened or was it likely to have happen in the life time of the disciples? No. We might be able to say that it has happened in our time, but not much before.
Back further, verse 5-8 tell of wars and rumors of wars that would happen before the end. Had these happened in the lifetime of the disciples? Perhaps. But the intensity and the extent here seems greater by far than what the disciples might have experienced prior to the Jewish Roman war or after. In addition there would be many who would claim to be the returning Messiah. That didn't happen in their lifetime.
Back further, verse 2 tells of the destruction of the temple. Did that happen? Yes. In 70 A.D.
But all of the other things do not fit in the period before or even closely after the destruction of the temple in 70 A.D. And Jesus makes it clear that it is not the destruction of the temple that will be the beginning of the end; it is the desecration of the temple. That, of course, requires that another temple be built. That hasn't happened yet.
So either we must declare that Jesus got almost all of it wrong or that he really was speaking of something more distantly future than the destruction of the temple, something beyond the lifetime of the disciples.
Who then are the "you?" What does "this generation" mean? Neither can mean the disciples there with Jesus on the Mount of Olives. It must mean others. It fits the whole passage much better if we understand that the "you" is addressing in a direct and personal way those in the future who will see these things happen.
"This generation" refers to the those who will see these things describe in the preceding verses happen. Could that have been the disciples? We have seen that it could not. It refers to the generation that will see the desecration of the temple, the appearance of false messiahs doing signs and wonders, the appearance of false messiahs claiming that they are the returned Jesus, and the completion of preaching the gospel in the whole world.
To rip "this generation" out of the context that identifies it and apply our own personal definition does damage to the plain reading of the passage.
But all that said, Jesus goes on in Mark to explain that no one knows the day or hour of his return. Therefore, be ready. Anticipate it, even as you are patient waiting for it. Otherwise it may surprise you and you will not be ready. (See the parable of the ten bridesmaids in Matthew 25.) That is why there is the tone of immediacy. No one knows, so everyone needs to be ready.